Most vehicles in operation today are powered by internal combustion (IC) engines. Internal combustion engines typically have multiple cylinders or other working chambers where combustion occurs. The power generated by the engine depends on the amount of fuel and air that is delivered to each working chamber. The engine must be operated over a wide range of operating speeds and torque output loads to accommodate the needs of everyday driving.
There are two basic types of IC engines; spark ignition engines and compression ignition engines. In the former combustion is initiated by a spark and in the latter by a temperature increase associated with compressing a working chamber charge. Compression ignition engines may be further classified as stratified charge compression ignition engines (e.g., most conventional Diesel engines, and abbreviated as SCCI), premixed charge compression ignition (PCCI), reactivity controlled compression ignition (RCCI), gasoline compression ignition engines (GCI or GCIE), and homogeneous charge compression ignition (HCCI). Some, particularly older, Diesel engines generally do not use a throttle to control air flow into the engine. Spark ignition engines are generally operated with a stoichiometric fuel/air ratio and have their output torque controlled by controlling the mass air charge (MAC) in a working chamber. Mass air charge is generally controlled using a throttle to reduce the intake manifold absolute pressure (MAP). Compression ignition engines typically control the engine output by controlling the amount of fuel injected (hence changing the air/fuel stoichiometry), not air flow through the engine. Engine output torque is reduced by adding less fuel to the air entering the working chamber, i.e. running the engine leaner. For example, a Diesel engine may typically operate with air/fuel ratios of 20 to 55 compared to a stoichiometric air/fuel ratio of approximately 14.5.
Fuel efficiency of internal combustion engines can be substantially improved by varying the engine displacement. This allows for the full torque to be available when required, yet can significantly reduce pumping losses and improve thermal efficiency by using a smaller displacement when full torque is not required. The most common method today of implementing a variable displacement engine is to deactivate a group of cylinders substantially simultaneously. In this approach the intake and exhaust valves associated with the deactivated cylinders are kept closed and no fuel is injected when it is desired to skip a combustion event. For example, an 8 cylinder variable displacement engine may deactivate half of the cylinders (i.e. 4 cylinders) so that it is operating using only the remaining 4 cylinders. Commercially available variable displacement engines available today typically support only two or at most three displacements.
Another engine control approach that varies the effective displacement of an engine is referred to as “skip fire” engine control. In general, skip fire engine control contemplates selectively skipping the firing of certain cylinders during selected firing opportunities. Thus, a particular cylinder may be fired during one engine cycle and then skipped during the next engine cycle and selectively skipped or fired during the next. In this manner, even finer control of the effective engine displacement is possible. For example, firing every third cylinder in a 4 cylinder engine would provide an effective reduction to ⅓rd of the full engine displacement, which is a fractional displacement that is not obtainable by simply deactivating a set of cylinders to create an even firing pattern.
Both spark ignition and compression ignition engines require emission control systems including one or more aftertreatment elements to limit emission of undesirable pollutants that are combustion byproducts. Catalytic converters and particulate filters are two common aftertreatment elements. Spark ignition engines generally use a 3-way catalyst that both oxidizes unburned hydrocarbons and carbon monoxide and reduces nitrous oxides (NOx). These catalysts require that on average the engine combustions be at or near a stoichiometric air/fuel ratio, so that both oxidation and reduction reactions can occur in the catalytic converter. Since compression ignition engines generally run lean, they cannot rely solely on a conventional 3-way catalyst to meet emissions regulations. Instead they use another type of aftertreatment device to reduce NOx emissions. These aftertreatment devices may use catalysts, lean NOx traps, and selective catalyst reduction (SCR) to reduce nitrous oxides to molecular nitrogen. The most common SCR system adds a urea/water mixture to the engine exhaust prior to the engine exhaust flowing through a SCR based catalytic converter. In the SCR element the urea breaks down into ammonia, which reacts with nitrous oxides in the SCR to form molecular nitrogen (N2) and water (H2O). Additionally, Diesel engines often require a particulate filter to reduce soot emissions.
To successfully limit engine emissions all aftertreatment system elements need to operate in a certain elevated temperature range to function more efficiently. Since 3-way catalysts are used in spark ignition engines where the engine air flow is controlled, it is relatively easy to maintain a sufficiently elevated engine exhaust temperature, in the range of 400 C, to facilitate efficient pollutant removal in a 3-way catalyst. Maintaining adequate exhaust gas temperature in a lean burn engine is more difficult, since exhaust temperatures are reduced by excess air flowing through the engine. There is a need for improved methods and apparatus capable of controlling the exhaust gas temperature of a lean burn engine over a wide range of engine operating conditions.